ZrMn2 -Type alloy partially substituted with cerium/praseodymium/neodymium and characterized by AB2 stoichiometry

ABSTRACT

A ternary alloy comprised of zirconium, manganese and a third element selected from cerium, praseodymium and neodymium is characterized in having AB 2  hexagonal crystal structure and stoichiometry. Members of a preferred class of compounds, represented by the empirical formula Zr x-1  M x  Mn 2  wherein &#34;x&#34; has a value between zero and about 0.3 and M is one of the selected metals, are particularly suitable for use as hydrogen storage materials.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Many types of intermetallic compounds are known for use as hydrogenstorage materials. Of particular interest herein are hydrogen storagematerials provided by three-component ZrMn₂ -type alloys in whichzirconium is partially substituted with cerium, praseodymium orneodymium, and which are characterized in having the C14 hexagonalcrystal structure and AB₂ stoichiometry.

2. State of the Art

A material suitable for storage of hydrogen must satisfy many demandingcriteria. In addition to large storage capacity for hydrogen, a hydrogenstorage material should absorb and desorb hydrogen quickly, preferablyat a pressure near one atmosphere, and the material should show aminimum of hysteresis effects during a hydrogen absorption/desorptioncycle.

Intermetallic compounds which have received much attention for use ashydrogen storage materials are provided by derivatives of ZrMn₂compounds, which compounds are characterized by hexagonal C14 crystalstructure and AB₂ stoichiometry. It is well known that the ZrMn₂ systemis capable of absorbing copious quantities of hydrogen, but thathydrides formed from the ZrMn₂ system are too stable to be of practicalsignificance. In search of improved ZrMn₂ -type systems, alloys havebeen prepared which contain other elements substituted for all or aportion of the zirconium, but with the AB₂ stoichiometry maintained inthe new alloy. For example, in Shaltiel et al, J. Less-Comm. Metals, 53,117-131 (1977), there are described changes in properties of AB₂Laves-phase ZrMn₂ -based compounds by substitution of manganese with a3d transition metal in accordance with the empirical formula Zr (Co_(x)M_(1-x))₂ and Zr (Fe_(x) M_(1-x))₂ wherein M=V, Cr, Mn and x is betweenzero and one. Other studies of partial substitution of zirconium inZrMn₂ alloys with titanium to form the hydrides of Ti_(1-x) Zr_(x) Mn₂pseudobinaries are described in Oesterreicher et al, Mat. Res. Bull, 13,83-88 (1978). In Fujii et al, J. Phys. Chem., 85, 3112-16 (1981),ternary alloys are described of the type Zr_(1-x) Ti_(x) Mn₂ wherein x=0to 0.5.

SUMMARY OF THE INVENTION

Improved hydrogen storage materials are provided by a ternary alloyconsisting of zirconium, manganese and a third element selected fromcerium, praseodymium and neodymium, which alloy is characterized inhaving the C14 hexagonal-type crystal structure and AB₂ stoichiometry. Arepresentative family of such alloys may be expressed by the empiricalformula

    Zr.sub.1-x M.sub.x Mn.sub.2                                (I)

wherein M=Ce, Pr, Nd and x has a value between zero and about 0.3.Alloys of particular interest within the scope of the formula I familyof compounds are as follows:

    Zr.sub.0.8 Ce.sub.0.2 Mn.sub.2

    Zr.sub.0.7 Ce.sub.0.3 Mn.sub.2

    Zr.sub.0.8 Nd.sub.0.2 Mn.sub.2                             (II)

Each of these specific alloys is characterized in having a desirablecombination of properties. That is, the alloys have fairly largehydrogen storage capacities along with the capability of forminghydrides less stable than ZrMn₂ systems, so as to make these alloyssuitable candidates for hydrogen storage materials at elevatedtemperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an equilibrium pressure-composition isotherm for the systemZr₀.8 Ce₀.2 Mn₂ --H₂, which is a representative ternary alloy hydride ofthe invention;

FIG. 2 is an equilibrium pressure-composition isotherm for the systemZr₀.7 Ce₀.3 Mn₂ --H₂, which is another representative ternary alloyhydride of the invention; and

FIG. 3 is a plot of the rates of absorption and desorption descriptionof hydrogen by the system Zr₀.8 Ce₀.2 Mn₂ at various temperatures.

DESCRIPTION OF PREFERRED EMBODIMENTS

A ternary alloy of the invention is characterized generally as aLaves-phase intermetallic compound composed of zirconium, manganese anda third element selected from the group consisting of cerium,praseodymium and neodymium in a C14 hexagonal crystal structure. Thecrystal structure for each of the specific compounds, of II above, ischaracterized by lattice parameters having approximate values in thefollowing ranges:

    a=5.00 A to 5.03 A

    c=8.20 A to 8.26 A

A general procedure for preparation of these ternary alloys follows.Weighed amounts of the constituents zirconium and the third element(cerium, or praseodymium, or neodymium) are placed in a copper boat forheating to a fusing temperature. Heating is accomplished typically bymeans of an r.f. 450 KHz induction heater. The copper boat is mountedinside a vacuum-tight quartz tube through which a stream of Ti-getteredargon passes during the heating period. Fusing of the constituents takesplace by heating a mixture of the constituents to about 1600° C. inabout two minutes, and holding at that temperature for about twominutes. Then the sample is cooled to room temperature in a period ofabout one minute and the hardened sample is turned over in the boat.Melting and cooling are repeated through four cycles, typically. Afterthe first cycle, a weighed amount of manganese, which includes an eightpercent stoichiometric excess over the amount of manganese desired inthe final sample is added to the fused sample of zirconium and theselected third element. Excess manganese is required to compensate forloss of manganese by evaporation. Then the sample is annealed for aperiod of about two hours at about 1000° C. X-ray diffraction analysisof the annealed sample typically shows a material consisting of a singlephase.

In order to activate the sample to make it suitable as a hydrogenstorage material, about two grams of the annealed sample is placed in astainless-steel pressure reactor vessel suitable for use in forming ahydride of the sample. The reactor is evacuated to a pressure of about10⁻³ Torr. Then pure hydrogen is pumped into the reactor to a pressureof about 40 to 50 atm., with the reactor vessel temperature initially atabout 25° C., until hydrogen is no longer absorbed by the sample.Usually, within two minutes of the time hydrogen is initially introducedinto the reactor, the reaction vessel temperature increases to about 50°C. Then the reactor is allowed to cool to room temperature over a periodof about 30 minutes, after which time the pressure within the reactor isusually about 45 atm. The pressure in the reactor is reduced to ambient,and then the sample is subjected to a pressure of about 10⁻³ Torr forabout 20 minutes in order for the sample to desorb substantially all ofthe previously-absorbed hydrogen.

In order to obtain a fully-activated hydrogen storage material, thesample is subjected to about 25 sorption-desorption cycles, underconditions as described for the activation procedure above. At the endof this activating period, there is obtained a repeatablepressure-composition profile. To obtain crystal structure data on thehydrides, a portion of the activated sample is hydrogenated to a knowncomposition in accordance with the previously-establishedpressure-composition isotherm. Then the hydrogenated sample is cooledquickly by quenching the sample boat (reactor) in liquid nitrogen, andrapidly pumping away remaining gaseous hydrogen. In accordance with thetechnique of Gualtieri et al. [J. Appl. Phys., 47, 3432 (1976)], a fewTorr of SO₂ is admitted to the reaction vessel to poison the surface ofthe sample, and thereby seal in the hydrogen. After the sample warms toroom temperature, X-ray diffraction data are obtained for the sample.

In order to demonstrate the preparation of the ternary alloys of theinvention and their hydrides, and to obtain data as to characteristicsand properties of the alloys, three ternary alloys were actuallyprepared in accordance with the aforementioned, generally-describedprocedures. Essential parameters such as constituent weights, meltingand annealing temperatures, lattice parameters and hydridingcharacteristics are summarized in Tables I-II. The cerium and neodymiumconstituent were 99.9 percent pure and used as obtained from NUCORCorp., Research Chemicals Div., Phoenix, AZ. The zirconium and manganeseconstituents were at 99.999 percent purity were used as obtained fromAlfa Products, Ventron Div., Danvers, MA.

The pressure-composition isotherms of FIGS. 1 and 2 for tworepresentative embodiments of the zirconium-cerium-manganese alloysystem of the invention, demonstrate important advantages of thisternary system over conventional ZrMn₂ systems. For example, at elevatedtemperatures these two alloy systems depicted can be hydrogenated anddehydrogenated at hydrogen pressures at about one atm.

The ternary alloys of the invention are also characterized by very rapidabsorption/desorption of hydrogen. As shown in FIG. 3, a condition of 90percent complete desorption or absorption of hydrogen can be obtained inless than about 30 seconds.

                                      TABLE I                                     __________________________________________________________________________    Preparation of Zr.sub.1-x M.sub.x Mn.sub.2 Ternary Alloys Wherein M = Ce,     Nd                                                                                                   Heat Treatment                                                     Amount of        Melting Cycles                                                                          Annealing                                                                             Sample Wt. Loss                Sample      Each Constituent (gm)                                                                    Melting  Melt Period                                                                          Temp                                                                              Period                                                                            During Preparation             No. Alloy   Zr  M  Mn  Temp (°C.)                                                                   No.                                                                              (Min)  (°C.)                                                                      (Hrs)                                                                             (gm)                           __________________________________________________________________________    I   Zr.sub.0.8 Ce.sub.0.2 Mn.sub.2                                                        1.500                                                                             0.560                                                                            2.439                                                                             ˜1500                                                                         5  3      ˜1000                                                                       2   0.011                          II  Zr.sub.0.7 Ce.sub.0.3 Mn.sub.2                                                        1.162                                                                             0.536                                                                            2.161                                                                             ˜1500                                                                         4  3      ˜1000                                                                       2   0.013                          III Zr.sub.0.8 Nd.sub.0.2 Mn.sub.2                                                        1.461                                                                             0.578                                                                            2.376                                                                             ˜1500                                                                         4  3      ˜1000                                                                       2   0.008                          __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________    Lattice Parameters and Hydrogen Absorption for Zr.sub.x-1 M.sub.x             Mn.sub.2 Ternary Alloys Wherein M = Ce, Nd                                                                     Change In Unit                                                                          Hydrogen Storage Capacity                                           Cell Volume In                                                                          ml H.sub.2 per                     Sample                                                                            Alloy/    Lattice Parameters                                                                      Unit Cell Volume                                                                       Forming Hydride                                                                         gm alloy      Activation           No. Alloy Hydride                                                                           a(A) c(A) V(A).sup.3                                                                             V/V %     (@ 40 atm)    Temp                 __________________________________________________________________________                                                             (°C.)         I   Zr.sub.0.8 Ce.sub.0.2 Mn.sub.2                                                          5.005                                                                              8.024                                                                              178      22.5                                             Zr.sub.0.8 Ce.sub.0.2 Mn.sub.2 H.sub.3.40                                               5.379                                                                              8.696                                                                              218                197           100                      Zr.sub.0.8 Ce.sub.0.2 Mn.sub.2 H.sub.3.25                                                                            188           130                      Zr.sub.0.8 Ce.sub.0.2 Mn.sub.2 H.sub.3.10                                                                            180           160                  II  Zr.sub.0.7 Ce.sub.0.3 Mn.sub.2                                                          4.998                                                                              8.195                                                                              177                                                       Zr.sub.0.7 Ce.sub.0.3 Mn.sub.2 H.sub.3.2                                                5.367                                                                              8.659                                                                              216      21.8      181           100                  III Zr.sub.0.8 Nd.sub.0.2 Mn.sub.2                                                          5.014                                                                              8.200                                                                              179      20.0                                             Zr.sub.0.8 Nd.sub.0.2 Mn.sub.2 H.sub.3.5                                                5.344                                                                              8.651                                                                              214                202           100                  __________________________________________________________________________

Although specific examples of the invention have been set forthhereinabove, it is not intended that the invention be limited solelythereto, but is to include all the variations and modifications fallingwithin the scope of the appended claims.

What is claimed is:
 1. A ternary alloy provided by elements in an atomicratio expressed by the formula:

    Zr.sub.1-x M.sub.x Mn.sub.2

wherein "M" is selected from the group consisting of cerium,praseodymium and neodymium, and wherein "x" has a value between zero andabout 0.3.
 2. The alloy of claim 1 expressed by the formula:

    Zr.sub.1-x Ce.sub.x Mn.sub.2

wherein "x" has a value between zero and about 0.3.
 3. The alloy ofclaim 1 expressed by the formula

    Zr.sub.1-x Nd.sub.x Mn.sub.2

wherein "x" has a value between zero and about 0.2.
 4. The hydrides ofcompounds of claim
 1. 5. A Laves phase intermetallic compound providedby a ternary alloy comprising zirconium, manganese and a third elementselected from the group consisting of cerium, praseodymium andneodymium, the intermetallic compound having a C14-type crystalstructure in which said third element is substituted for zirconium so asto maintain substantially ZrMn₂ stoichiometry, the crystal structurecharacterized by lattice parameters of

    a=4.99 A to 5.03 A

    c=8.19 A to 8.26 A


6. The ternary alloy of claim 5 wherein the third element is cerium andthe crystal structure is characterized by lattice parameters of

    a=4.99 A to 5.03 A

    c=8.19 A to 8.26 A


7. The ternary alloy of claim 5 wherein the third element is neodymiumand the crystal structure is characterized by lattice parameters of

    a=4.99 A to 5.03 A

    c=8.19 A to 8.26 A.


8. The ternary alloy of claim 5 wherein the third element is cerium andthe atomic ratios of the three elements fall within ranges expressed bythe formula

    Zr.sub.1-x Ce.sub.x Mn.sub.2

wherein "x" has a value between zero and about 0.3.
 9. The ternary alloyof claim 8 wherein "x" has a value of about 0.2.
 10. The ternary alloyof claim 8 wherein "x" has a value of about 0.3.
 11. The hydrides ofternary alloys of claim
 8. 12. The ternary alloy of claim 5 wherein thethird element is neodymium and the atomic ratios of the three elementsfall within ranges expressed by the formula

    Zr.sub.1-x Nd.sub.x Mn.sub.2

wherein "x" has a value between zero and about 0.2.
 13. The ternaryalloy of claim 12 wherein "x" has a value of about 0.2.
 14. The hydridesof ternary alloys of claim
 12. 15. A method for forming a hydride of analloy comprising the steps of(a) preparing a ternary alloy of zirconium,manganese and a third element selected from cerium, praseodymium andneodymium, said alloy capable of storing hydrogen as hydride of thealloy; (b) subjecting the alloy to hydrogen so that hydrogen is absorbedinto the alloy.
 16. The method of claim 15 further characterized by thestep of decomposing the hydride to cause desorption of hydrogen from thealloy.
 17. An alloy having the empirical formula

    Zr.sub.0.8 Ce.sub.0.2. Mn.sub.2


18. An alloy having the empirical formula

    Zr.sub.0.7 Ce.sub.0.3 Mn.sub.2.


19. An alloy having the empirical formula

    Zr.sub.0.8 Nd.sub.0.2 Mn.sub.2.


20. A hydride having the empirical formula

    Zr.sub.0.8 Ce.sub.0.2 Mn.sub.2 H.sub.3.4.


21. A hydride having the empirical formula

    Zr.sub.0.8 Ce.sub.0.2 Mn.sub.2 H.sub.3.25.


22. A hydride having the empirical formula

    Zr.sub.0.8 Ce.sub.0.2 Mn.sub.2 H.sub.3.1.


23. A hydride having the empirical formula

    Zr.sub.0.7 Ce.sub.0.3 Mn.sub.2 H.sub.3.2.


24. A hydride having the empirical formula

    Zr.sub.0.8 Nd.sub.0.2 Mn.sub.2 H.sub.3.5.